Grease a 9x13 pan with butter or cooking spray.

Melt the butter in a large pot over medium heat. Keep cooking, swirling occasionally, until it foams and turns golden brown with a nutty smell, about 3-4 minutes. Watch it closely, it goes from brown to burnt fast.

Add the marshmallows and salt. Stir until completely melted and smooth.

Remove from heat and quickly fold in the Rice Krispies until evenly coated.

Press into the prepared pan with buttered hands or a spatula. Don't pack them too tight or they'll be hard.

Let cool for 30 minutes, then cut into squares.